In an example of image formation process in the electrophotographic method, an electrostatic image is formed on a photoreceptor by giving uniformly charge and imagewise exposure thereto. The electrostatic image is developed by a developer containing a toner to form a toner image, and the toner image is transferred to a paper and fixed to form a visual image. On the other hand, a cleaning member arranged on the surface of photoreceptor so as to be contacted with pressure to the surface removes the toner which is not transferred to the paper and remained on the photoreceptor.
In the fixing process of such the image forming procedure, the fixing image is usually formed by the use of a heat-roller fixing device.
There is a drawback in the fixing by the heat-roller device that an offset problem tends to raise. The offset problem is caused by transferring a part of the molten toner to the surface of the heat-roller, the transferred toner is transferred back to the next paper so as to contaminate the image.
A technique has been known to prevent the offset problem, in which a particle of low molecular weight polypropylene is contained in the colored particle for giving a mold-releasing ability of the toner.
The low molecular weight polypropylene particle has a high mole-releasing ability itself and is able to be sharply fused when the particle is heated. Therefore, when the toner comprising such the colored particle is used, the low molecular weight polypropylene particle is rapidly fused at the fixing treatment by the heat-roller and the surface of the toner particle is covered by the fused low molecular weight polypropylene. As a result of that, the interface energy between the toner particle and the surface of heat-roller is lowered and the mold releasing ability of the toner is made higher.
However, a part of the low molecular weight polypropylene particles can not be included in the colored particles since the miscibility of the low molecular weight polypropylene particle with the binder resin of the colored particle is not so high. Accordingly, low molecular weight polypropylene particles freely existing outside the colored particles, hereinafter referred to free polypropylene particle, are formed.
When the toner containing the free polypropylene particles is used in the image forming process, problems are raised such as that the charging ability of the toner is made unstable and toner filming on the photoreceptor tends to be formed.
Moreover, a problem that a black spot like contamination is formed on the image when the diameter of the free polypropylene particle is large.
Furthermore, a problem is raised that the presence of the free polypropylene particles causes lowering in the flowing ability of the toner and the life of the developer is shortened.
On the other hand, when a toner with no free polypropylene particle or a wax-free toner is used in the image forming process, a sufficient transferring ability cannot be obtained since the particles are not present which fill the role of spacer for lowering the adhesive force between the toner particle and the photoreceptor.